Recent developments have occurred in the field of very small switches having moving liquid metal-to-metal contacts and that are operated by an electrical impulse. That is, they are actually small latching relays that individually are SPST or SPDT, but which can be combined to form other switching topologies, such as DPDT. (Henceforth we shall, as is becoming customary, refer to such a switch as a Liquid Metal Micro Switch, or LIMMS.) With reference to FIGS. 1-4, we shall briefly sketch the general idea behind one class of these devices. Having done that, we shall advance to the topic that is most of interest to us, which is an improved technique for forming the needed channels and cavities of such switches fabricated on a substrate.
Refer now to FIG. 1A, which is a top sectional view of certain elements to be arranged within a cover block 1 of suitable material, such as glass. The cover block 1 has within it a closed-ended channel 7 in which there are two small movable distended droplets (12, 13) of a conductive liquid metal, such as mercury. The channel 7 is relatively small, and appears to the droplets of mercury to be a capillary, so that surface tension plays a large part in determining the behavior of the mercury. One of the droplets is long, and shorts across two adjacent electrical contacts extending into the channel, while the other droplet is short, touching only one electrical contact. There are also two cavities 5 and 6, within which are respective heaters 3 and 4, each of which is surrounded by a respective captive atmosphere (10, 11) of a suitable gas, such as N2. Cavity 5 is coupled to the channel 7 by a small passage 8, opening into the channel 7 at a location about one third or one fourth the length of the channel from its end. A similar passage 9 likewise connects cavity 6 to the opposite end of the channel. The idea is that a temperature rise from one of the heaters causes the gas surrounding that heater to expand, which splits and moves a portion of the long mercury droplet, forcing the detached portion to join the short droplet. This forms a complementary physical configuration (or mirror image), with the large droplet now at the other end of the channel. This, in turn, toggles which two of the three electrical contacts are shorted together. After the change the heater is allowed to cool, but surface tension keeps the mercury droplets in their new places until the other heater heats up and drives a portion of the new long droplet back the other way. Since all this is quite small, it can all happen rather quickly; say, on the order of a millisecond, or less. The small size also lends itself for use amongst controlled impedance transmission line structures that are part of circuit assemblies that operate well into the microwave region.
To continue, then, refer now to FIG. 1B, which is a sectional side view of FIG. 1A, taken through the middle of the heaters 3 and 4. New elements in this view are the bottom substrate 2, which may be of a suitable ceramic material, such as that commonly used in the manufacturing of hybrid circuits having thin film, thick film or silicon die components. A layer 14 of sealing adhesive bonds the cover block 1 to the substrate 2, which also makes the cavities 5 and 6, passages 8 and 9, and the channel 7, each moderately gas tight (and also mercury proof, as well!). Layer 14 may be of a material called CYTOP (a registered trademark of Asahi Glass Co., and available from Bellex International Corp., of Wilmington, Del.). Also newly visible are vias 15-18 which, besides being gas tight, pass through the substrate 2 to afford electrical connections to the ends of the heaters 3 and 4. So, by applying a voltage between vias 15 and 16, heater 3 can be made to become very hot very quickly. That in turn, causes the region of gas 10 to expand through passage 8 and begin to force long mercury droplet 12 to separate, as is shown in FIG. 2. At this time, and also before heater 3 begins to heat, long mercury droplet 12 physically bridges and electrically connects contact vias 19 and 20, after the fashion shown in FIG. 1C. Contact via 21 is at this time in physical and electrical contact with the small mercury droplet 13, but because of the gap between droplets 12 and 13, is not electrically connected to via 20.
Refer now to FIG. 3A, and observe that the separation into two parts of what used to be long mercury droplet 12 has been accomplished by the heated gas 10, and that the right-hand portion (and major part of) the separated mercury has joined what used to be smaller droplet 13. Now droplet 13 is the larger droplet, and droplet 12 is the smaller. Referring to FIG. 3B, note that it is now contact vias 20 and 21 that are physically bridged by the mercury, and thus electrically connected to each other, while contact via 19 is now electrically isolated.
The LIMMS technique described above has a number of interesting characteristics, some of which we shall mention in passing. They make good latching relays, since surface tension holds the mercury droplets in place. They operate in all attitudes, and are reasonably resistant to shock. Their power consumption is modest, and they are small (less than a tenth of an inch on a side and perhaps only twenty or thirty thousandths of an inch high). They have decent isolation, are reasonably fast with minimal contact bounce. There are versions where a piezo-electrical element accomplishes the volume change, rather than a heated and expanding gas. There also exist certain refinements that are sometimes thought useful, such as bulges or constrictions in the channel or the passages. Those interested in such refinements are referred to the Patent literature, as there is ongoing work in those areas. See, for example, U.S. Pat. No, 6,323,447 B1.
To sum up our brief survey of the starting point in LIMMS technology that is presently of interest to us, refer now to FIG. 4. There is shown an exploded view 32 of a slightly different arrangement of the parts, although the operation is just as described in connection with FIGS. 1-3. In particular, note that in this arrangement the heaters (3, 4) and their cavities (5, 6) are each on opposite sides of the channel 7. Another new element to note in FIG. 4 is the presence of contact electrodes 22, 23 and 24. These are (preferably thin film) depositions of metal that are electrically connected to the vias (19, 20 and 21, respectively). They not only serve to ensure good ohmic contact with the droplets of liquid metal, but they are also regions for the liquid metal to wet against, which provides some hysteresis in the pressures required to move the droplets. This helps ensure that the contraction caused by the cooling (and contraction) of the heated (and expanded) operating medium does not suck the droplet back toward where it just came from. The droplets of liquid metal are not shown in the figure.
If contact electrodes 22-24 are to be produced by a thin film process, then they will most likely need to be fabricated after any thick film layers of dielectric material are deposited on the substrate (as will occur in connection with many of the remaining figures). This order of operations is necessitated if the thick film materials to be deposited need high firing temperatures to become cured; those temperatures can easily be higher than what can be withstood by a layer of thin film metal. Also, if the layer of thin film metal is to depart from the surface of the substrate and climb the sides of a channel, then it might be helpful if the transition were not too abrupt.
LIMMS devices can be combined into structures that have several switches under one cover block. Thus, the xe2x80x9cfloor planxe2x80x9d of a LIMMS device can range from relatively simple to fairly complex. Ease of manufacture and control of tolerances are important considerations for any fabrication technique that is to be practiced for volume production. The presently known techniques for creating individual cover blocks of etched glass or ceramic material require that they be first formed and then positioned, adhered and then hermetically sealed. It is not that the prior art does not work, but it becomes awkward, inefficient and expensive when production volume increases, especially when the LIMMS themselves, or the arrangement of LIMMS within an assembly, become complex. It would therefore be advantageous if there were an inexpensive, controllable and scalable way of forming the needed channels and cavities within LIMMS devices of increasing complexity. What to do?
An attractive solution to the problem of efficient fabrication of the channels and cavities in a LIMMS device is to form them from a layer of thick film dielectric material deposited on a substrate, and then cover the layer with a thin cover plate of ceramic or perhaps glass. The layer of dielectric material may be patterned using established thick film techniques. It is essentially as easy to do this for complicated multi-LIMMS arrangements, and for LIMMS of complicated internal structure, as it is for simple arrangements and structures, and good dimensional control can be achieved. The dielectric layer is itself its own hermetic seal against the substrate, and readily lends itself to the formation of the additional hermetic seal needed between itself and the cover plate.
This plan depends upon the use of a suitable dielectric material, which must be strong, adheres well to the substrate, is impervious to contaminants, is capable of being patterned, and if also desired, which can be metalized for soldering. It should also have well controlled and suitable properties as a dielectric. Given a choice, a lower dielectric constant (K) is preferable over a higher one. Suitable thick film dielectric materials that may be deposited as a paste and subsequently cured include the KQ 150 and KQ 115 thick film dielectrics from Heraeus and the 4141 A/D thick film compositions from DuPont.